This invention relates to a microwave circular polarization analyzer and more particularly to one that samples orthogonal linearly polarized components of an incident radiation field.
Theory suggests that the emission from certain classes of microwave devices should be in a single circularly polarized state. Study of emission from a high-gain 35-GHz free-electron laser (FEL) amplifier utilizing a right handscrew helical wiggler magnetic field and operating in the fundamental TE.sub.11 .degree. mode gave rise to the desire to test a theoretical prediction that the emission should be left circularly polarized. Various devices that might be used to perform this test were considered. Each such device had drawbacks in terms of cost and complexity, and required specialized fabrication as well as careful adjustment and calibration.
The literature lists a variety of circular-polarization diplexers that could seemingly permit this determination. In a circular-polarization diplexer, a microwave signal entering one (circular) port is divided between two output ports depending on the fraction of the signal in each circular polarization. One possible configuration is the use of a quasi-optical quarter-wave plate or a waveguide quarter-wave section (e.g. a section of elliptical waveguide) to convert circular to linear polarization, followed by a linear polarization analyzer, such as a fin-line polarization analyzer. Another alternative is a complex waveguide device called a turnstile junction, with four rectangular ports, a circular port, and several coaxial matching pins. By shorting two of the rectangular arms at precise fractions of a wavelength from the circular port, this junction will act as a circular polarization analyzer without requiring a quarter-wave section, and the right and left circularly polarized components of the signal in the circular arm will separate into the remaining two rectangular arms. Each of these approaches has drawbacks in terms of cost and complexity, requiring specialized fabrication as well as careful adjustment and calibration.
If the condition of a single input port is relaxed to allow two separate matched input ports, to be positioned to sample a uniform portion of a radiation field, such as the center of the antenna pattern of a microwave horn driven by a TE.sub.1n .degree. mode, simpler circular polarization analyzers are possible. One might use a circularly polarized helical beam antenna with a rectangular feed; a matched pair of such antennas, one left and one right circularly polarized would serve this function. As an alternative, a simple circular-polarization analyzer can be constructed whose principal components are a hybrid coupler, which is readily available commercially, and two 45.degree. waveguide twists, which are easily fabricated from straight waveguide. This device will operate without special adjustment and calibration over the bandwidth of the hybrid coupler (typically at least 10%).